POSIX_FADVISE(2)          (2019-03-06)           POSIX_FADVISE(2)

          posix_fadvise - predeclare an access pattern for file data

          #include <fcntl.h>

          int posix_fadvise(int fd, off_t offset, off_t len

     Feature Test Macro Requirements for glibc (see

              _POSIX_C_SOURCE >= 200112L

          Programs can use posix_fadvise() to announce an intention to
          access  file  data in a specific pattern in the future, thus
          allowing the kernel to perform appropriate optimizations.

          The advice applies to a (not  necessarily  existent)  region
          starting at offset and extending for len bytes (or until the
          end of the file if len is 0) within the file referred to  by
          fd.   The  advice  is  not binding; it merely constitutes an
          expectation on behalf of the application.

          Permissible values for advice include:

               Indicates that the application has no  advice  to  give
               about its access pattern for the specified data.  If no
               advice is given for an open file, this is  the  default

               The application expects to access  the  specified  data
               sequentially  (with  lower  offsets  read before higher

               The specified data will be accessed in random order.

               The specified data will be accessed only once.

               In kernels before 2.6.18,  POSIX_FADV_NOREUSE  had  the
               same semantics as POSIX_FADV_WILLNEED.  This was proba-
               bly a bug; since kernel 2.6.18, this flag is a no-op.

               The specified data will be accessed in the near future.

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               POSIX_FADV_WILLNEED initiates a nonblocking read of the
               specified  region  into  the page cache.  The amount of
               data read may be decreased by the kernel  depending  on
               virtual  memory load.  (A few megabytes will usually be
               fully satisfied, and more is rarely useful.)

               The specified data will not be  accessed  in  the  near

               POSIX_FADV_DONTNEED attempts to free cached pages asso-
               ciated  with the specified region.  This is useful, for
               example, while streaming large files.   A  program  may
               periodically  request  the  kernel  to free cached data
               that has already been used, so that more useful  cached
               pages are not discarded instead.

               Requests to discard partial pages are ignored.   It  is
               preferable   to   preserve  needed  data  than  discard
               unneeded data.  If the application requires  that  data
               be  considered for discarding, then offset and len must
               be page-aligned.

               The implementation may  attempt  to  write  back  dirty
               pages  in the specified region, but this is not guaran-
               teed.  Any unwritten dirty pages will not be freed.  If
               the  application wishes to ensure that dirty pages will
               be released, it should call  fsync(2)  or  fdatasync(2)

          On success, zero is returned.  On error, an error number  is

               The fd argument was not a valid file descriptor.

               An invalid value was specified for advice.

               The specified file descriptor refers to a pipe or FIFO.
               (ESPIPE  is  the  error  specified by POSIX, but before
               kernel version 2.6.16, Linux returned  EINVAL  in  this

          Kernel support first appeared in Linux 2.5.60; the  underly-
          ing  system call is called fadvise64().  Library support has
          been provided since glibc version 2.2, via the wrapper func-
          tion posix_fadvise().

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          Since Linux 3.18, support for the underlying system call  is
          optional,     depending    on    the    setting    of    the
          CONFIG_ADVISE_SYSCALLS configuration option.

          POSIX.1-2001, POSIX.1-2008.  Note that the type of  the  len
          argument  was  changed  from size_t to off_t in POSIX.1-2001

          Under Linux, POSIX_FADV_NORMAL sets the readahead window  to
          the     default     size    for    the    backing    device;
          POSIX_FADV_SEQUENTIAL     doubles     this     size,     and
          POSIX_FADV_RANDOM  disables  file readahead entirely.  These
          changes affect the  entire  file,  not  just  the  specified
          region  (but  other  open  file handles to the same file are

          The contents of the kernel buffer cache can be  cleared  via
          the /proc/sys/vm/drop_caches interface described in proc(5).

          One can obtain a snapshot of which pages of a file are resi-
          dent  in the buffer cache by opening a file, mapping it with
          mmap(2), and then applying mincore(2) to the mapping.

        C library/kernel differences
          The name of  the  wrapper  function  in  the  C  library  is
          posix_fadvise().   The  underlying  system  call  is  called
          fadvise64() (or, on some architectures, fadvise64_64()); the
          difference  between  the  two is that the former system call
          assumes that the type of the len argument is  size_t,  while
          the latter expects loff_t there.

        Architecture-specific variants
          Some architectures require 64-bit arguments to be aligned in
          a  suitable  pair  of  registers (see syscall(2) for further
          detail).  On  such  architectures,  the  call  signature  of
          posix_fadvise() shown in the SYNOPSIS would force a register
          to be wasted as padding between the fd and offset arguments.
          Therefore,  these architectures define a version of the sys-
          tem call that orders the arguments suitably, but  is  other-
          wise exactly the same as posix_fadvise().

          For example, since Linux 2.6.14, ARM has the following  sys-
          tem call:

              long arm_fadvise64_64(int fd, int advice,
                                    loff_t offset, loff_t len);

          These architecture-specific  details  are  generally  hidden
          from applications by the glibc posix_fadvise() wrapper func-
          tion, which invokes  the  appropriate  architecture-specific

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          system call.

          In kernels before 2.6.6, if len was  specified  as  0,  then
          this  was interpreted literally as "zero bytes", rather than
          as meaning "all bytes through to the end of the file".

          fincore(1),  mincore(2),  readahead(2),  sync_file_range(2),
          posix_fallocate(3), posix_madvise(3)

          This page is part of release 5.10  of  the  Linux  man-pages
          project.   A  description  of the project, information about
          reporting bugs, and the latest version of this page, can  be
          found at https://www.kernel.org/doc/man-pages/.

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